getting hit

Why do children automatically know just the wrong, and I mean wrong place to hit their dads? it is inevitable that at least once a day I will get hit in the groin by one of my daughters who in turn will laugh hysterically. The day I got my vasectomy my now 5 year old got mad at me and balled her little fist and made very direct contact with my very "tender" nuts. yeah I dropped like a sack of flour. I got hit with a lacrosse ball shot at goal that shattered my cup and it was n ot nearly as painful. My wife tells me not to give such a "good" reaction, but it is hard. I always ask her if she knows what it feels like to get hit in the balls? (knowing she does not, since she does not have man parts) And she will say no. I then said that it is hard not to cry out, and one time shed a tear, when it happens. I have tried, trust me I have tried but it does not work. There is no way at all that you cannot react, but how do they instinctively know where it hurts daddy the most. It is like once you become a dad your nuts are a magnet for punishment.